Active-matrix liquid-crystal electro-optical device s using TFTs are known. These TFTs are made of an amorphous or polycrystalline semiconductor. A TFT of either P-channel or N-channel type is used for each one pixel. Generally, an n-channel TFT is connected with each pixel. A typical example of this configuration is shown in FIG. 2.
FIG. 2 is a schematic equivalent circuit of a conventional liquid-crystal electro-optical device of the above-described kind. The liquid-crystal portion of one pixel is indicated by 22. An n-channel TFT 21 is connected in series with this portion. Pixels of this structure are arranged in rows and columns. Generally, a very large number of pixels such as 640.times.480 or 1280.times.960 pixels are arranged. In this figure, only a matrix of 2.times.2 pixels is shown for simplicity. Signals are applied from peripheral circuits 26 and 27 to the pixels to selectively turn on and off the pixels. If the switching characteristics of the TFTs are good, it is generally possible to obtain a high contrast from this liquid-crystal electro-optical device by time-sharing techniques even at high duty factors.
However, in such a liquid-crystal electro-optical device fabricated in practice, the output signal from each TFT, i.e. the input voltage VLC 20 (hereinafter referred to as the liquid-crystal potential) to the liquid crystal, often fails to take "1" when it should take "1" (High). Conversely, the voltage sometimes fails to take "0" when it should take "0" (Low). This phenomenon occurs because the TFT acting as a switching device applying a signal to the pixel assumes asymmetric states when it is turned on and off.
The liquid crystal 22 is intrinsically insulative in operation. When the TFT is OFF, the liquid-crystal potential VLC is in a floating condition. This liquid crystal 22 is a capacitor in terms of an equivalent circuit. The potential VLC is determined by the electric charge stored in this capacitor. This charge leaks when the resistance RLC 24 of the liquid crystal is small or when dust or ionic impurities are present in the liquid crystal. If a resistance RGS 25 is produced between the gate electrode and the input or output terminal of the TFT 21 because of pinholes unintentionally occurred in the gate-insulating film of the TFT 21, then the charge leaks from this location. As a result, the potential VLC 20 takes a halfway value.
In a liquid-crystal display comprising a panel having 200 thousand to 500 million pixels, as many TFTs exist. Therefore, the above-described problem takes place. This makes it impossible to accomplish a high production yield. Generally, the liquid crystal 22 consists of a twisted-nematic liquid crystal. To orient the liquid crystal, a rubbed orientation film is formed on each electrode. Static electricity is produced by the rubbing and induces a weak dielectric breakdown in the TFT. As a result, a leakage between the TFT and an adjacent pixel or conductive interconnect occurs, or the gate-insulating film is weak enough to permit leakage. It is quite important for the active-matrix liquid-crystal electro-optical device that the liquid-crystal potential be maintained at the initial value throughout one frame. The actual situation is that this requirement is not always satisfied because of numerous defects.
Where the liquid-crystalline material is a ferro-electric liquid crystal, it is necessary to set the injection current to a large value. For this purpose, the current margin of the TFT must be made large by increasing the dimensions of it.